Suppose that Symfony 2.7 version hasn't been released yet and you want to create
a new project to test its features. First, install the Composer
package manager. Then, open a command console, enter your project's directory and
execute the following command:

Suppose again that Symfony 2.7 hasn't been released yet and you want to upgrade
an existing application to test that your project works with it.

First, open the composer.json file located in the root directory of your
project. Then, edit the value of the version defined for the symfony/symfony
dependency as follows:

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{"require":{"symfony/symfony":"2.7.*@dev"}}

Finally, open a command console, enter your project directory and execute the
following command to update your project dependencies:

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$ composer update symfony/symfony

If you prefer to test a Symfony beta version, replace the "2.7.*@dev" constraint
by "2.7.0-beta1" to install a specific beta number or 2.7.*@beta to get
the most recent beta version.

After upgrading the Symfony version, read the Symfony Upgrading Guide
to learn how you should proceed to update your application's code in case the new
Symfony version has deprecated some of its features.

Tip

If you use Git to manage the project's code, it's a good practice to create
a new branch to test the new Symfony version. This solution avoids introducing
any issue in your application and allows you to test the new version with
total confidence: